In the simplest form, an organic electroluminescent (EL) device is comprised of organic electroluminescent media disposed between first and second electrodes. The first and second electrodes serve as an anode for hole injection and a cathode for electron injection. The organic electroluminescent media supports recombination of holes and electrons that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED display device that is useful as a display such as, for example, a television, computer monitor, cell phone display, personal digital assistant display, music player display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic display, or they can be made to produce multiple colors such as a three-pixel red, green, blue (RGB) display. For purposes of this disclosure, a pixel is considered the smallest individual unit, which can be independently stimulated to produce light. As such, the red pixel, the green pixel, and the blue pixel are considered as three distinct pixels.
The simplest pixelated OLED displays are driven in a passive matrix configuration. In a passive matrix, the organic EL material is sandwiched between two sets of electrodes, arranged orthogonally as rows and columns. An example of a passive matrix driven OLED display is described in U.S. Pat. No. 5,276,380. This approach to producing a pixelated display, however, has several disadvantages. First, only a single row (or column) is illuminated at any given time. Therefore, in order to achieve the desired average brightness for a given frame of video, the row should be illuminated to an instantaneous brightness equal to the desired average brightness multiplied by the number of rows. This results in higher voltages and reduced long term reliability compared to a situation where the pixels are capable of being lit continuously for the entire frame. Second, the combination of high instantaneous current and electrodes that are long and narrow, and therefore have high resistance, results in significant voltage drops across the device. These variations in voltage across the display adversely affect brightness uniformity. These two effects become worse as the size of the display and number of rows and columns are increased, thereby limiting the usefulness of passive matrix designs to relatively small, low resolution displays.
To resolve these problems and produce higher performance devices, OLED displays driven by active matrix (AM) circuitry have been shown. In an active matrix configuration, each pixel is driven by multiple circuit elements such transistors, capacitors, and signal lines. This circuitry permits the pixels of multiple rows to remain illuminated simultaneously, thereby decreasing the required peak brightness of each pixel. Examples of active matrix drive OLED displays are shown in U.S. Pat. Nos. 5,550,066, 5,684,365, 6,281,634, 6,456,013, 6,501,466, 6,535,185, 6,737,800 and 6,392,340, 6,753,654 and 6,798,145 and U.S. Patent Application Numbers 2005/0218798A1 and 2003/0216100A1.
The active matrix circuitry is commonly achieved by forming thin film transistors (TFT's) from thin layers of semiconductor material, such as silicon, deposited onto the substrate. The two most common types of TFT's are amorphous silicon type TFT's and polysilicon type TFT's. These TFT's are commonly fabricated using thin film deposition, photolithographic patterning, and etching techniques known in the art. Each layer of the TFT is built up using one or more, and often all three, of these techniques. Amorphous silicon TFT's are constructed by using a silicon layer with an amorphous structure. As such they tend to have low performance in terms of the their ability to conduct and are typically limited to n-type transistors, also known as NMOS. Polysilicon type TFT's are fabricated by annealing amorphous silicon at elevated temperatures to crystallize the silicon layer into a poly-crystalline state. As such, polysilcon type TFT's have better performance and can also be fabricated into both n-type (NMOS) and p-type (PMOS) transistors. A common method of annealing polysilicon type TFT's is by excimer laser annealing (ELA). However, the additional processing steps required to anneal the polysilicon and fabricate both NMOS and PMOS type transistors typically result in such polysilicon type devices having a high manufacturing cost.
In addition to the silicon layers, several conductor and insulator layers are typically deposited and patterned to complete the TFT's as well as the wiring and other components such as capacitors. Commonly, two different conductor layers are used in the fabrication of the TFT wiring. The conductor layers are used to form the gate terminal and the source and drain terminal connections to the TFT's. In addition, these two conductor layers also form an orthogonal grid of wiring in a row direction and a column direction. Since two spaced apart conductor layers are used with at least one insulator layer in between, the row wiring and the column wiring can be formed and electrically isolated from one and other. Typically, data signal lines are formed in one of these two conductor layers while row select lines are formed in the other conductor layer. This allows the pixels to be selected, for example row by row, while the brightness intensity data is loaded from the column direction. These conductor layers are typically formed of highly conductive materials such as chromium, molybdenum, aluminum, aluminum alloys such as aluminum neodymium, or the like.
Since OLED devices require a constant current supply to sustain illumination, active matrix OLED devices typically provide a power line electrically connected to a voltage source to supply current to one or more rows or columns of pixels. Current is then regulated between this power line and the lower electrode of the organic light emitting diode by one or more transistors, referred to as power transistors or drive transistors. The circuit is completed by electrically connecting the upper electrode of the organic light emitting diode to a second voltage source, such as a ground voltage. This upper electrode is frequently common to all the pixels and does not require precision, pixel level patterning or alignment.
In prior art OLED displays, the power line is formed in either of the two previously described conductor layers. The signal lines formed in each of such layers are patterned into separate, electrically isolated, features during a photolithographic patterning and an etching step. By forming the power line in one of these two metal layers which are already required to form the mesh of data lines and select lines, the power lines can be formed without any additional photolithographic patterning steps. Therefore, cost to produce the display can be kept low. The prior art power lines have been arranged in either a row direction or a column direction and can be arranged to supply electrical current to one or more of such rows or columns of pixels. Such power lines are frequently formed of metals such as aluminum, aluminum alloys such as aluminum neodymium, chromium, or molybdenum. Examples of various arrangements of these power lines can be found in U.S. Pat. Nos. 6,522,079, 6,919,681, and 6,771,028.
Prior art displays also typically employ another conductor layer formed over the two previously described conductor layers to form the lower electrodes for the OLED element. This conductor layer is, in some examples, transparent to allow transmission of the light emission and is therefore constructed of transparent conductive materials such as indium tin oxide (ITO), indium zinc oxide (IZO), aluminum zinc oxide (AZO), or the like. However, these transparent conductive materials are not as highly conductive as the materials used for the lower conductor layers, which do not have the requirement of optical transparency. Therefore, this transparent conductor layer is not well suited for carrying current across long distances of many pixels.
As display sizes increase, for example from small displays such as are useful for cellular telephones or digital cameras to large displays such as are useful for monitors or televisions, the length of these power lines and the total amount of electrical current being carried by the power lines both increase. This may result in a large electric resistance that cause large voltage variations across the power lines from the center to the edge of the display even when highly conductive materials such as aluminum are employed. These voltages variations can adversely affect the luminance uniformity of the display as well as result in wasted power consumption. One method of reducing this resistance is to increase the width of the power line as described in U.S. Pat. No. 6,762,564. Another approach to improving the current supply across the panel as described in U.S. Pat. Nos. 6,724,149 and 6,714,178 is to provide a first set of power lines in the same conductor layer as the select lines in the row direction and a second set of power lines in the same conductor layer as the data lines in the column direction and connect them together to form a grid or matrix. However, both of these approaches are limited in effectiveness since the area available for light emission is reduced when features in these two opaque conductor layers are added or increased in size. Any such reduced area for light emission requires the organic light emitting diode to be driven at higher currents, which increases power consumption and reduces display lifetime. U.S. Pat. No. 6,714,178 also suggests providing yet another highly conductive conductor layer specifically for the purpose of forming discrete power lines. This approach, however, has a disadvantage in that additional manufacturing steps are required, thereby increasing manufacturing cost. Therefore, a new OLED display device that can provide a power supply to the pixels with reduced resistance across the display while maintaining low manufacturing cost is desired.